Method of creating a cast automotive product having an improved critical fracture strain

ABSTRACT

The present invention provides a casting having increased crashworthiness including an aluminum alloy of about 6.0 wt % to about 8.0 wt % Si; about 0.12 wt % to about 0.25 wt % Mg; less than or equal to about 0.35 wt % Cu; less than or equal to about 4.0 wt % Zn; less than or equal to about 0.6 wt % Mn; and less than or equal to about 0.15 wt % Fe, wherein the cast body is treated to a T5 or T6 temper and has a tensile strength ranging from 100 MPa to 180 MPa and has a critical fracture strain greater than 10%. The present invention further provides a method of forming a casting having increased crashworthiness.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/553,236, filed Oct. 26, 2006, which claims the benefit of U.S.Provisional Application No. 60/731,046, filed Oct. 28, 2005, eachapplication of which is hereby incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates to aluminum casting alloys that aresuitable for automotive applications. Specifically, the presentinvention relates to Al—Si—Mg casting alloys having a crash performancethat is suitable for automotive applications.

BACKGROUND OF THE INVENTION

Aluminum alloys are highly desirable for vehicle frame constructionbecause they offer low density, good strength and corrosion resistance.Moreover, aluminum alloys can be employed to improve vehicle framestiffness and performance characteristics. Moreover, it is believed thatan aluminum vehicle frame retains the strength and crashworthiness thatis typically associated with much heavier, conventional steel framevehicle designs.

An important consideration for aluminum automotive body structuresincludes crashworthiness in conjunction with reducing the overallvehicle weight and/or improving vehicle performance. For the automotiveapplications, crashworthiness reflects the ability of a vehicle tosustain some amount of collision impact without incurring unacceptabledistortion of the passenger compartment or undue deceleration of theoccupants. Upon impact, the structure should deform in a prescribedmanner; the energy of deformation absorbed by the structure shouldbalance the kinetic energy of impact; the integrity of the passengercompartment should be maintained; and the primary structure should crushin such a manner as to minimize the occupant deceleration.

The demand for higher crash performance of automotive aluminum castcomponents has greatly increased, particularly with respect to bodystructures, including but not limited to: brackets, nodes (e.g. A-Post,B-Post, C-Post, etc.), crashboxes, crossmembers, subframes, and enginecradles; etc. However, the most common aluminum cast alloy, A356 haspoor crushability even in T6 temper. One characterization ofcrashworthiness is critical fracture strain (CFS) as developed by Yeh.See J. R. Yeh, “The Development of an Aluminum Failure Model forCrashworthiness Design”, Report No. 99-016, 1999-03-11. The criticalfracture strain (CFS) or A356-T6 is approximately 5-6%. Typically, thecritical fracture strain (CFS) required for a crash sensitive componentsand applications is on the order of 10% or greater.

Therefore, a new alloy and heat treatment are needed for producing castcomponents with a balanced strength and crashworthiness.

SUMMARY OF THE INVENTION

The present invention provides an Al—Si—Mg base alloy suitable forgravity or low pressure permanent mold, high pressure die casting, orsand mold casting, having a tensile strength and critical fracturestrain (CFS) suitable for automotive applications, including but notlimited to frame components.

In one embodiment, the invention comprises of an inventive Al—Si—Mg basealloy for gravity or low pressure permanent mold, high pressure diecasting, sand mold casting or like casting processes, wherein a castingproduced by the inventive Al—Si—Mg alloy is suitable for F, T5 or T6tempers in achieving a yield strength ranging from approximately 100 MPato approximately 180 MPa, an elongation ranging from 10% to 20%, and acritical fracture strain (CFS) of at least 10%. Broadly, the compositionof the Al—Si—Mg alloy of the present invention consists essentially of:

about 6.0 wt % to about 8.0 wt % Si;

about 0.12 wt % to about 0.25 wt % Mg;

less than or equal to about 0.35 wt % Cu;

less than or equal to about 4.0 wt % Zn;

less than or equal to about 0.6 wt % Mn;

less than or equal to about 0.15 wt % Fe; and

a balance of aluminum and impurities.

In one embodiment of the present invention, an Al—Si—Mg alloy isprovided for Vacuum Riserless Casting (VRC)/Pressure Riserless Casting(PRC), permanent mold or sand mold casting comprising from about 6.5 wt% to about 7.5 wt % Si; from about 0.12 wt % to about 0.20 wt % Mg; lessthan about 0.15 wt % Mn, preferably being less than about 0.05 wt % Mn;and less than about 0.10 wt % Fe, wherein the balance comprises Al andimpurities.

In another embodiment of the present invention, an Al—Si—Mg alloy isprovided for high pressure die casting or Alcoa Vacuum Die Casting(AVDC) and comprises from about 6.5 wt % to about 7.5 wt % Si; fromabout 0.12 wt % to about 0.20 wt % Mg; from about 0.5 wt % to about 0.6wt % Mn; and Fe less than 0.10 wt %, wherein the balance comprises Aland impurities. The increased Mn content reduces soldering of the moldduring the casting process, wherein the Mn content reduces the tendencyof the casting to stick to the mold.

In one preferred embodiment, the Si content ranges from 6.5 wt. % to 7.5wt. % and the Mg content ranges from about 0.12 wt. % to about 0.19 wt.%.

In another aspect of the present invention, a process incorporating theinventive Al—Si—Mg alloy composition produces a casting having a tensilestrength comparable to A356 and having a critical fracture strain (CFS)greater than A356, when cast and treated to a T5 or T6 temper. A356typically has a composition of Al-7Si-0.35Mg, and when heat treated to aT5 or T6 temper has a tensile yield strength (TYS) ranging from 190 MPato 240 MPa and a critical fracture strain (CFS) ranging from 5% to 6%.

This alloy heat treatment of the inventive process depends on the rangeof performance targeted, and covers natural aging, T4, T5, T6 and T7.

The casting complexity allows the shape of the product to reach thestiffness target performance that is comparable to or higher than thatof existing steel design.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a stress v. strain curve.

FIG. 2 (side view) depicts a three point bending test.

FIGS. 3 a-3 d pictorially represent bend test specimens treated to T5temper.

FIGS. 4 a-4 d pictorially represent bend test specimens treated to T6temper.

FIG. 5 depicts a plot of the bending performance of high pressure diecastings composed of an aluminum alloy within the scope of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a casting alloy composition having abalanced strength and crashworthiness for automotive castings. Broadly,the inventive casting alloy comprises:

6.0 wt %-8.0 wt % Si,

0.12 wt %-0.25 wt % Mg,

less than 0.35 wt % Cu,

less than 4.0 wt % Zn,

less than 0.6 wt % Mn,

less than 0.15 wt % Fe,

at least one silicon modifier,

at least one grain refining element, and

a balance of aluminum and impurities.

All component percentages herein are by weight percent (wt %) unlessotherwise indicated. When referring to any numerical range of values,such ranges are understood to include each and every number and/orfraction between the stated range minimum and maximum. A range of about6.0 wt %-8.0 wt % Si, for example, would expressly include allintermediate values of about 6.1, 6.2 and 6.3 wt %, all the way up toand including, 8.0 wt % Si. The term “impurities” denotes anycontamination of the melt, including leaching of elements from thecasting apparatus. Allowable ranges of impurities are less than 0.05 wt% for each impurity constituent and 0.15 wt % for total impuritycontent.

The aluminum casting alloy of the present invention has a Mgconcentration that increases critical fracture strain (CFS) in thepresence of excess silicon. Although silicon increases castability bypromoting high fluidity and low shrinkage in the cast alloy, increasedsilicon content results in the formation of silicon particles resultingin a casted body having low crashworthiness. In one embodiment, the Mgconcentration of the present invention is selected to produce the properproportion of Mg₂Si precipitates in the metal solution, withoutexcessive precipitate formation at the grain boundaries, in providing acasting alloy that when subjected to precipitate hardening results in atensile strength suitable for automotive applications and a criticalfracture strain (CFS) greater than or equal to 10%.

The critical fracture strain (CFS) is one value used to characterizecrashworthiness. The CFS may be derived from an engineering stress v.strain curve that is generated from a sample of an alloy being tested.The stress v. strain curve may be determined using quasi-static freebend testing (ASTME190), quasi-static crush testing, quasi-static axialcrush, or quasi-static three-point bend.

Using a stress v. strain-curve, as depicted in FIG. 1, the engineeringstrain at maximum load (ε_(m)) 10, the engineering stress at maximumload (δ_(m)) 15 and the engineering stress at the fracture load (δ_(f))20 may be determined and then entered into the following equation toprovide the engineering thinning strain (ε_(l, eng)):

ε_(l,eng)=ε_(m,)/2+(1−(ε_(m,)/2))×(1−(δ_(f)/δ_(m)))

The engineering thinning strain (ε_(l, eng)) is then used to derive thecritical fracture strain (CFS) through the following equation:

CFS=−ln(1−ε_(l,eng))

Generally, materials having a high CFS value perform better under largedeformation than materials having a low CFS value. Typically, priormaterials and alloy compositions encounter severe cracking during crushtesting when characterized by a CFS value lower than 10%.

In one embodiment, the inventive crashworthy alloy composition comprisesof an Al—Si—Mg base alloy for gravity or low pressure permanent mold, orsand mold casting with the following composition ranges (all in weightpercent):

6.0 wt % to 8.0 wt % Si,

0.12 wt % to 0.25 wt % Mg,

less than 0.35 wt % Cu,

less than 4.0 wt % Zn,

less than 0.6 wt Mn, and

less than 0.15 wt % Fe.

In another embodiment, the inventive crashworthy alloy compositioncomprises of an Al—Si—Mg alloy for high pressure die casting, whereinthe manganese wt % may be increased to 0.55 for preventing diesoldering, in which the increased manganese content decreases thelikelihood that the alloy sticks to the mold during the casting process.

The Cu level of the casting alloy may be increased to increase thealloy's strength performance. But, increasing the Cu content to greaterthan 0.35 wt % may have a disadvantageous effect on ductility. The Zncontent may be increased to reduce the alloy's sensitivity tosolidification rate. It may be particularly useful to increase the Zncontent for casting applications of greater thicknesses, where thesolidification rate significantly differs from the center most portionof the casting in comparison to the portions of the casting that are indirect contact with the mold surface.

The inventive alloy composition may further include Si modifiers andgrain refiners. In one example, a Si modifier may be included in theabove alloy composition in an amount less than or equal to 0.02 wt %,wherein the Si modifier is selected from the group consisting of Ca, Na,Sr, and Sb. In one example, grain refiners may include titaniumdiboride, TiB₂ or titanium carbide, TiC. If titanium diboride isemployed as a grain refiner, the concentration of boron in the alloy maybe in a range from 0.0025 wt. % to 0.05 wt. %. Likewise, if titaniumcarbide is employed as a grain refiner, the concentration of carbon inthe alloy may be in the range from 0.0025 wt. % to 0.05 wt. %. Typicalgrain refiners are aluminum alloys containing TiC or TiB₂.

One aspect of the present invention, is a process for forming a castingfrom the above alloy composition, including a specified heat treatment.This alloy heat treatment depends on the range and type of performancetargeted, and covers natural aging, T4, T5, T6 and T7 temper. In oneembodiment, a T5 heat treatment typically includes an artificial agingtreatment at a temperature of about 150° C. to 250° C. for a time periodranging from a % a hour to 10 hours.

Typically, a T6 treatment includes a three stage treatment beginningwith a solution heat treatment of 450° C. to 550° C. for a time periodranging from approximately ½ hour to approximately 6 hours. Followingsolution heat treatment, a quench process is conducted using air quench,forced air quench, liquid mist quench or liquid submersion quench. It isnoted that the quench rate may be increased, so long as the casting isnot distorted or residual stresses are not induced into the castingsurface, or the quench rate may be decreased, so long as the percentageof precipitating elements remaining in the supersaturated solution isnot so adversely affected to substantially reduce the casting'sstrength. Following quench, the casting is then artificially aged topeak strength, wherein the aging process is similar to that used inprocessing to T5 temper.

A T4 treatment typically includes a three stage treatment similar to T6,but in the final stage of the T4 treatment the casting is naturallyaged, whereas in T6 heat treatments the casting is artificially aged atan elevated temperature. More specifically, the first stage of the T4treatment is preferably a solution heat treatment of 450° C. to 550° C.for a time period ranging from approximately ½ hour to approximately 6hours. Following solution heat treatment, a quench process is conductedusing air quench, forced air quench, liquid mist quench or liquidsubmersion quench. Following quench, the casting is then naturally aged.

The T7 heat treatment is similar to T6 and also comprises solution heattreatment, quench and aging process steps. Opposed to T6 temper in whichthe artificial aging step is conducted to peak strength, the agingprocess step of the T7 heat treatment continues until overaging, whereinin some embodiments overaging of the castings while having a negativeeffect on strength advantageously increases corrosion resistance.

The casting complexity allows the shape of the product to reach astiffness target performance that is comparable or higher than that ofexisting steel design.

The combination of the targeted Al—Si—Mg alloy and the manufacturingprocess provide castings suitable for crash applications.

In one embodiment, Vacuum Riserless Casting/Pressure Riserless Casting(VRC/PRC) of the present alloy provides very flexible process parametersthat permit casting of any part, wherein the cast parts may have a wallthickness of 3 mm or higher, in which the castings may be solid orhollow.

The Vacuum Riserless Casting (VRC)/Pressure Riserless Casting (PRC)process is suitable for mass production of high integrity aluminumautomotive chassis and suspension components. VRC/PRC is a low pressurecasting process, in which in some embodiments the pressure may be on theorder of 6.0 Psi. In VRC/PRC, a mold is positioned over a hermeticallysealed furnace and the casting cavity is connected to the melt by feedtubes(s). Melt is drawn into the mold cavity by applying a pressure tothe furnace through the application of an inert gas, such as Ar. Aconstant melt level is maintained in the furnace of the VRC/PRCapparatus, avoiding back-surges that are sometimes experienced in themore traditional low pressure system.

Multiple fill tubes (stalks) provide ideal metal distribution in themold cavity. Multiple fill points combined with close coupling betweenthe mold and melt surface allows lower metal temperatures, minimizeshydrogen and oxide contamination and provides maximum feeding ofshrinkage-prone areas in the casting. The multiple fill tubes also allowmultiple yet independent cavities in a mold. Carefully sequenced thermalcontrols quickly solidify castings from extreme back to fill tubes,which then function as feed risers.

In another embodiment of the present invention, the alloy may beutilized in die casting applications, preferably being high pressure diecasting, such as Alcoa Vacuum Die Casting (AVDC). A more detaileddiscussion of AVDC is described in U.S. Pat. No. 5,246,055, titled“Vacuum Die-Casting Machine with Apparatus and Method for ControllingPressure Behind the Piston”, which is incorporated in its entirety byreference.

Die casting is a process in which molten metal is injected at highvelocity and pressure into a mold (die) cavity preferably made ofhigh-quality steel. Under ideal conditions the metal does not solidifybefore the cavity is filled.

Alcoa Vacuum Die Casting (AVDC) is a form of high pressure die casting,wherein AVDC preferably evacuates the entire die cavity and shot systemas it draws melt under vacuum into the shot tube and then injects itunder high pressure into the die. AVDC substantially reduces, preferablyeliminating, the atmosphere in the shot systems and die cavity. Toaccomplish this, the shot and die cavity system are preferably wellsealed to avoid drawing in ambient air when under high vacuum. Incomparison to typical die casting, AVDC produces a vacuum at least oneorder of magnitude greater than the highers vacuum that can possibly beprovided by typical die casting. If the atmosphere in the shot systemand die cavity are essentially eliminated there can be little to no airavailable to be admixed and entrapped in the molten metal during cavityfill.

In comparison to low pressure Vacuum Riserless Casting (VRC)/PressureRiserless Casting (PRC) providing a pressure on the order of 6 Psi, AVDCproduces a high pressure that is orders of magnitude greater than thepressure produced by VRC/PRC casting operations, whereas in someembodiments AVDC provides a pressure on the order of 10 Ksi or greater.

In one embodiment, the alloy of the present invention when utilized inVRC/PRC castings unexpectable provides comparable bending performance tocastings formed by high pressure die castings, such as AVDC castings,whereas an observable advantage in bending performance is typicallypresent in alloys cast by high pressure die castings when compared tocastings prepared by VRC/PRC.

AVDC typically provides a higher solidification rate than VRC/PRC, andtherefore results in castings having a smaller grain size, smallerparticle size and smaller dendritric spacing, which all contribute togreater ductility and greater bending performance. Comparable bendingperformance in castings formed using VRC/PRC casting technology can beunexpectedly obtained, by utilizing the casting alloy of the presentinvention, in which the Mg content has been tailored to reduce Mg₂Siparticle formation, in combination with reducing the thickness of thecastings by utilizing sand cores to provide wall thicknesses of lessthan 6 mm, preferably being 4.0 mm, and being as thin as 2.0 mm, so longas castability is not adversely affected.

The following examples are provided to further illustrate the presentinvention and demonstrate some advantages that arise therefrom. It isnot intended that the invention be limited to the specific examplesdisclosed.

Example 1 Critical Fracture Strain

Castings representing three alloy compositions within the inventivealloy composition were prepared from direction solidification molds andpermanent molds. One example of permanent mold systems includes VacuumRiserless Casting/Pressure Riserless Casting. The composition of eachalloy sample tested are provided in Table I, in which sample A wasprepared on a laboratory scale from a directional solidification moldand samples B and C were prepared on a production scale from VacuumRiserless Casting/Pressure Riserless Casting molds utilizing a sand coreto provide a hollow casting having a wall thickness on the order ofapproximately 4 mm.

TABLE I Compositions of the alloy Alloy composition Si Mg Cu Zn Fe A7.05 0.1 0 2.57 0.02 B 7.03 0.16 0.35 0 0.06 C 7.01 0.177 0 0.0025 0.087

Following casting the samples where then air cooled. Regardless of thecasting process, the solidification rates of the cast structures fromthe directionally solidified molds and the permanent molds weresubstantially equal. The cast structures where then heat treated to F,T5 or T6 temper. T5 heat treatment comprised artificial aging at atemperature of about 150° C. to 250° C. for a time period ranging from a½ hour to 10 hours. T6 treatment comprised a solution heat treatment of450° C. to 550° C. for a time period ranging from approximately ½ hourto approximately 6 hours, liquid quench, and artificial aging at atemperature of 150° C. to 250° C. for a time period ranging from a ½hour to 10 hours. Following heat treatment the casting where thenmachined into test samples and subjected to tensile testing inaccordance with ASTM B557. (See ASTM B557: Tension testing wrought andcast aluminum and magnesium alloy products). The ultimate tensilestrength, yield tensile strength, and elongation of the alloys listed inTable I were then recorded. A stress-strain curve was provided for eachalloy listed in Table I, from which the critical fracture strain (CFS)was determined. The critical fracture strength (CFS), ultimate fracturesstrength (UTS), tensile yield strength (TYS), and Elongation was thenrecorded in Table II.

TABLE II Tensile properties and CFS of the alloy Test TYS UTS E CFSAlloy Scale Temper (MPa) (MPa) (%) (%) A Lab T5 122 204 18 — B Lab T5150 232 10 — C Plant F 90 200 14 — C Plant T5 144 218 10 7 C Plant T5110 190 14 10 C Plant T6 166 256 14 14 C Plant T6 135 227 16 24 C PlantT6 180 270 10 9

The results indicate that an adequate combination of strength andcrashworthiness can be achieved in Al—Si—Mg alloys for automotiveapplications by controlling the alloy composition and heat treatment.High ductility and high critical fracture strain are generally observedin the invented alloy. Both T5 and T6 tempers increase the strength ofthe alloy. However, the decrease of ductility and critical fracturetoughness observed with increasing strength is slower in T6 temper whencompared to T5 temper.

Example II Visual Inspection for Cracking in Bend Testing

VRC/PRC castings were then prepared using alloy composition C, as listedin Table 1, treated to T5 and T6 temper, and then subjected to bendtesting and visual inspection for cracking. The castings were preparedfrom (VRC)/(PRC) casting technology as described above using a sand coreto provide a wall thickness on the order of approximately 4.0 mm. Thetemperature for the T5 and T6 heat treatments was similar to thedescription of the heat treatments described in Example 1. The timeperiod for heat treatment was varied to test the bending performance forvarying strengths of the specified alloys. Table III illustrates thecasting samples and heat treatments.

TABLE III Compositions of the Alloy and Temper Test TYS CFS Sample ScaleTemper (MPa) (%) 1 Plant T5 144 7 2 Plant T5 110 10 3 Plant T6 135 24 4Plant T6 180 9

Samples 1 and 2 were treated to T5 temper, in which the aging time ofsample 1 was greater than the aging time of sample 2. Samples 3 and 4were treated to a T6 temper, in which the aging time of sample 4 wasgreater than the aging time of sample 3.

Following heat treatment, the samples were machined into test plates 30a, 30 b a having a length of approximately 60 mm, a width ofapproximately 30 mm, and a thickness on the order of approximately 2 mm.

The Tensile Yield Strength (TYS) and the Critical Fracture Strength(CFS) were measured in accordance with the method discussed in Example1, and are listed here for comparative purposes with respect to thevisual bend test.

Referring to FIG. 2, the bend test was similar to three point bendtesting, in which the base points 25 have a cylindrical geometry and theloading nose 35 has a wedge geometry. During actuation the loading wedgecontacts the centerline of the test plate 30 a, 30 b (the test plateprior to deformation is designated by reference number 30 a, the testplate following deformation is designated by reference number 30 b)deforming the test plate 30 a, 30 b against the base points. Thedeformation of the test plate 30 a, 30 b continues until angle α1 ofapproximately 15° is formed at the apex A1 of displacement D1 of thedeformed plate 30 b relative to a plane p1 passing through the apex A1and being parallel to the plate 30 a prior to deformation.

Following bend testing the samples were visually inspected for cracking.The incidence of cracking indicates a sample that is less suitable forcrash applications than a sample that does not display cracking.

Referring to FIGS. 3 a and 3 b, pictorial representations are providedfor Sample 1, in which the test plate was deformed to an angle of 15°with visual cracking on the bending surface. Specifically, Sample 1comprised alloy composition C at T5 temper with an aging time to providea TYS on the order of 144 MPa.

Referring to FIGS. 3 c and 3 d, pictorial representations are providedfor Sample 2, in which the test plate was deformed to an angle of 15°without substantial visual cracking on the bending surface.Specifically, Sample 2 comprised alloy composition C at 15 temper withan aging time to provide a TYS on the order of 110 MPa, wherein theaging time used to prepare Sample 2 is lesser than the aging time ofSample 1. Comparing Samples 1 and 2 indicates that increasing agingtime, while increasing TYS, also increases the incidence of crackingresulting in decreased crashworthiness.

Referring to FIGS. 4 a and 4 b, pictorial representations are providedfor Sample 3, in which the test plate was deformed to an angle of 15°without substantial visual cracking on the bending surface.Specifically, Sample 3 comprised alloy composition C at T6 temper withan aging time to provide a TYS on the order of 135 MPa.

Referring to FIGS. 4 c and 4 d, pictorial representations are providedfor Sample 4, in which the test plate was deformed to an angle of 15°with visual cracking on the bending surface. Specifically, Sample 4comprised alloy composition C at T6 temper with an aging time to providea TYS on the order of 180 MPa, wherein the aging time used to prepareSample 4 is greater than the aging time used to prepare Sample 3.Comparing Samples 3 and 4 indicates that increasing aging time, whileincreasing TYS, decreases crashworthiness.

It is further noted that a correlation exists between cracking duringbend test and critical fracture strains less than 10%. Specifically,with reference to Table III, FIGS. 3 a-3 d and FIGS. 4 a-4 d, it isnoted that visual cracking is noted in bend tests for alloys and heattreatments characterized by a critical fracture strain less than 10%,such as samples 1 and 4.

Example III Bending Performance Comparision of High Pressure Die Castingand Vacuum Riserless Casting (VRC)/Pressure Riserless Casting(PRC)

Test samples representing the inventive alloy composition were preparedusing Vacuum Riserless Casting/Pressure Riserless Casting and HighPressure Die Casting. The test samples were heat treated, machined intotest plates 30 a, 30 b, and subjected to bend testing. The alloycomposition was composed of approximately 9.0-10.0 wt % Si,approximately 0.2 wt ° A) Mg, approximately 0.5-0.6 wt % Mn, and abalance of aluminum and impurities.

VRC/PRC test samples were cast using a sand core to provide a wallthickness on the order of approximately 4.0 mm. Die cast test sampleshad a thickness on the order of approximately 2.5 mm.

The test samples formed using VRC/PRC casting processes where heattreated to T6 temper. T6 treatment comprised a solution heat treatmentof 450° C. to 550° C. for a time period ranging from approximately ½hour to approximately 6 hours, liquid quench, and artificial aging at atemperature of 150° C. to 250° C. for a time period ranging from a ½hour to 10 hours.

The test samples formed using high pressure die casting, were also heattreated to peak strength including slightly higher aging temperatures ofabout 250° C. to 300° C. Following heat treatment the test samples weremachined into test plates 30 a, 30 b having a thickness on the order ofapproximately 2.0 mm.

A bend test was conducted on the VRC/PRC castings and the high pressuredie castings. Referring to FIG. 2, the bend test is similar to threepoint bend test, in which the base points 25 have a cylindrical geometryand the loading nose 35 has a wedge geometry. During actuation theloading nose 35 contacts the centerline of the test plate 30 a, 30 bdeforming the test plate 30 a, 30 b against the base points. Thedeformation of the test plate 30 a, 30 b continues until the incidenceof cracking was visually confirmed. The force applied to the casting andthe angle of bending at the incidence of cracking.

The force applied to the casting is the force applied through theloading nose 35. Referring to FIG. 2, the failure angle is the angle a1at which cracking was visually verified. The failure angle a1 ismeasured from the apex A1 of displacement D1 of the deformed plate 30 brelative to the plane p1 passing through the apex A1 and being parallelto the plate 30 a prior to deformation. The failure angle a1 and forcefor the VRC/PRC castings and the high pressure die castings formed ofthe alloy of the present invention is recorded in Table IV.

TABLE IV COMPARISON OF HIGH PRESSURE DIE CASTING AND VRC/PRC BENDINGPERFORMANCE Force Failure Sample (N) Angle Casting Method 1 1386 44VRC/PRC 2 1369 46 VRC/PRC 3 986 52 VRC/PRC 4 895 51 VRC/PRC 5 1018 53High Pressure Die Casting 6 1044 54 High Pressure Die Casting 7 1039 50High Pressure Die Casting 8 1039 53 High Pressure Die Casting 9 1059 53High Pressure Die Casting 10 882 45 High Pressure Die Casting 11 813 40High Pressure Die Casting 12 1008 51 High Pressure Die Casting 13 928 48High Pressure Die Casting 14 940 47 High Pressure Die Casting

As indicated in Table IV, the bending performance of VRC/PRC castingswas comparable to the bending performance of high pressure die castings.Specifically, VRC/PRC castings (samples #1-4) were recorded having anangle failure of 44 to 52°, and high pressure die castings (samples#6-14) were recorded having an angle failure of 40 to 54°.

FIG. 5 depicts a plot graphically representing the bending performanceof the high pressure die casting samples recorded in Table IV (sample#'s 6-14), wherein the y-axis depicts the force (N) applied through theloading nose 35, and the displacement (mm) represents the displacementat the apex of the plate 30 b resulting from the force of the loadingnose. Each plotted data line represents a test sample, wherein the datalines have been shifted along the x-axis for clarity. The area aboveeach curve represents the force and displacement that results in visualcracking during bending and the area below each curve represents theforce and displacement that does not result in visual cracking.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A method of casting an aluminum alloy into a cast product having animproved critical fracture strain, the method comprising: providing amelt having an alloy comprising: about 6.0 wt % to about 7.8 wt % Si;about 0.10 wt % to about 0.18 wt % Mg; less than or equal to about 0.35wt % Cu; less than or equal to about 0.6 wt % Mn; less than or equal toabout 0.15 wt % Fe; and, a balance of aluminum and impurities; castingthe melt into a cast body; and, heat treating the cast body; wherein,the heat treating includes solution heating, quenching, and aging; and,the cast body has a critical fracture strain of at least about 10%. 2.The method of claim 1, wherein the heat treating comprises processing toa T6 temper.
 3. The method of claim 1, wherein the processing comprisesaging for a time of less than about 10 hours.
 4. The method of claim 1,wherein the processing comprises a solution heating at 450° C. to 550°C. for about a ½ hour to about 6 hours, quenching, and aging.
 5. Themethod of claim 1, wherein the aging includes treating at a temperatureof about 150° C. to about 250° C. for about ½ hour to about 10 hours. 6.The method of claim 1, wherein the processing comprises a solution heattreatment of 450° C. to 550° C. for a time period ranging fromapproximately a ½ hour to approximately 6 hours, quench, and aging. 7.The method of claim 1, wherein the Mn content is less than 0.15 wt %,and the casting includes using a vacuum riserless casting/pressureriserless process.
 8. The method of claim 1, wherein the castingincludes using a sand core in combination with a vacuum riserlesscasting/pressure riserless process to provide a wall thickness of 61 mmor less.
 9. The method of claim 1, wherein the cast body is in T6 temperand has a tensile yield strength ranging from 100 MPa to 180 MPa. 10.The method of claim 1, wherein the Mn content is less than about 0.05 wt% Mn.
 11. The method of claim 1, wherein the critical fracture strain ofthe heat treated alloy composition exceeds 10%, the tensile yieldstrength exceeds 100 MPa, and the heat treatment process comprises T6 orT7 to further provide an elongation that exceeds 14%.
 12. The method ofclaim 1, the melt having about 7.0 wt % Si and from about 0.10 wt % toabout 0.18 wt % Mg.
 13. The method of claim 1, the melt having about0.12 wt % Mg and from about 6.0 wt % to about 7.8 wt % Si.
 14. A methodof casting an aluminum alloy product, the method comprising: providing amelt having an alloy comprising: about 6.0 wt % to about 7.8 wt % Si;about 0.10 wt % to about 0.18 wt % Mg; less than or equal to about 0.35wt % Cu; less than or equal to about 0.15 wt % Mn; less than or equal toabout 0.15 wt % Fe; and, a balance of aluminum and impurities; castingthe melt into a cast body; and heat treating the cast body; wherein, theheat treating includes solution heating, quenching, and aging; and, theheat treated, cast body has a critical fracture strain of at least about10% and a tensile yield strength of at least 100 MPa.
 15. The method ofclaim 14, wherein the heat treated, cast body has an elongation thatexceeds 14%.
 16. The method of claim 14, wherein the Mn content is lessthan about 0.05 wt % Mn.
 17. The method of claim 14, the melt havingabout 7.0 wt % Si and from about 0.10 wt % to about 0.18 wt % Mg. 18.The method of claim 14, the melt having about 0.12 wt % Mg and fromabout 6.0 wt % to about 7.8 wt % Si.
 19. A method of creating aheat-treated, cast automotive product having a critical fracture strainof greater than 10% and an elongation of greater than 14%, the methodcomprising: forming an automotive casting from a melt that includes analuminum alloy, the melt comprising: about 6.0 wt % to about 7.8 wt %Si; about 0.10 wt % to about 0.18 wt % Mg; less than or equal to about0.35 wt % Cu; less than or equal to about 0.15 wt % Mn; less than orequal to about 0.15 wt % Fe; and a balance of aluminum and impurities;and, heat treating the automotive casting using a T6 process; wherein,the heat treating includes solution heating, quenching, and aging; and,the heat treated, automotive casting has a critical fracture strain ofat least about 10%, a tensile yield strength of at least 100 MPa, and anelongation of at least about 14%.
 20. The method of claim 19, the melthaving about 7.0 wt % Si and from about 0.10 wt % to about 0.18 wt % Mg.21. The method of claim 19, the melt having about 0.12 wt % Mg and fromabout 6.0 wt % to about 7.8 wt % Si.